Carbon dioxide (CO2), H2S, COS, mercaptans, and aromatic hydrocarbons, are examples of impurity gases contained in natural gas, etc., and these impurity contents tend to gradually increase in recent years in association with decreasing of quality of natural gas, etc., obtained from gas wells. Accordingly, it is necessary to purify the natural gas, etc., by removing the impurity gases therefrom in order to produce a product gas.
As a method for removing H2S contained in an impurity gas (acid gas), on the other hand, one which utilizes a Claus reaction is available. In the Claus reaction, a part of H2S in a gas is oxidized to be SO2, and the SO2 obtained is reacted with residual H2S to react and recover elementary sulfur substances, thereby removing H2S in the gas.
Also, a method is known in which the rate of removing sulfur components is increased by hydrogenating residual SO2, which is contained in an off-gas after the Claus reaction, to be H2S in the presence of catalyst, and returning the obtained H2S to a Claus reaction apparatus.
As mentioned above, various components other than H2S are contained as impurity components accompanied with a natural gas, etc., and the following problems may occur when such impurity gases are separated from the natural gas, etc., and are introduced into a Claus reaction apparatus in order to remove sulfur components contained therein:                1. the rate of removing the sulfur components is decreased since a large amount of components other than H2S, for instance CO2, is contained in an impurity gas (acid gas), which reduces the concentration of H2S and decreases the reaction rate in the Claus reaction; and        2. aromatic hydrocarbons (benzene, toluene, xylene, etc., (BTX)) contained in an impurity gas (acid gas) are incompletely combusted and soot is generated, and hence the sulfur recovered may be contaminated by the soot, thereby decreasing the quality of the sulfur, and also the Claus catalyst layer may be clogged by the soot.        
As a method for subjecting a hydrogen sulfide containing gas including a large amount of CO2, for example, one which is disclosed in Japanese Examined Patent Application, Second Publication No. Sho 63-17488 is conventionally known.
In this method, a part of H2S containing gas including 20 vol. % or more of CO2 is supplied to a Claus plant, and the rest of the gas is made to contact with an absorbent which selectively absorbs H2S, and H2S which is regenerated and separated from the absorbent is returned to the Claus plant.
However, using this method, it is not possible to sufficiently solve the above-mentioned problems.
As a method for solving the problem, one is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-28837.
In this method, as shown in FIG. 1, a residual impurity gas (acid gas) containing CO2, H2S, mercaptans, and BTX, etc., which is obtained after purifying a product natural gas from a raw natural gas, is separated to a concentrated gas including H2S as a main gas and to a residual gas including CO2 as a main gas, mercaptans, and BTX, in an absorption tower 41 and a regeneration tower 42.
Then, the concentrated gas is transferred to a Claus sulfur recovering device 43, and a majority of H2S is recovered as elementary sulfur in the device 43. An off-gas (exhaust gas) from the Claus sulfur recovering device 43 is transferred to a heating furnace 44 together with the above-mentioned residual gas to be heated, and then the heated gas is transferred to a hydrogenation reactor 45. In the hydrogenation reactor 45, sulfur compounds, such as SO2, mercaptans, and sulfur vapor, contained in the heated gas is reduced to H2S in the presence of a reduction catalyst.
The obtained H2S is separated in an absorption tower 46, and this is returned to the Claus sulfur recovering device 43 via the regeneration tower 42.
This method of prior application has advantages such as a high removing rate of sulfur from sulfur compounds contained in an impurity gas (acid gas), prevention of clogging in a catalyst layer of the Claus sulfur recovering device 43, and high quality of recovered elementary.
However, it was found that even this method has the following elements that should be improved.
That is, since the residual gas and the off-gas are introduced into the above-mentioned heating furnace 44 at the same time and heated, soot may be generated due to BTX contained in the residual gas, and the generated soot may flow into the hydrogenation reactor 45 used in the next step so as to deactivate the reduction catalyst or clog the catalyst layer.
The cause of the problem is found to be direct contact of the residual gas with the flame of a combustion burner of the heating furnace 44, and the temperature at that time is 700° C. or higher. Accordingly, the problem may be solved if the residual gas does not make direct contact with the flame, or the gas is not heated to the temperature of 700° C. or higher. In order to achieve these, the length of the heating furnace 44 may be increased so that the residual gas is introduced to a low temperature portion separated away from the flame. However, in this manner, a new problem will be created in which the size of the device must be increased.
Also, when a relatively large amount of 1–10 vol. % of mercaptans is contained in the residual gas, mercaptans may be adsorbed by the reduction catalyst in the hydrogenation reactor 45, and this prevents sufficient desulfurization of the mercaptans. Hence, the recovery rate of the sulfur may not be increased in the overall process.
Accordingly, a first object of the present invention is to prevent the generation of soot in the heating furnace, which is a problem associated with the method for removing sulfur compounds disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-28837, and to prevent increase in size of the heating furnace.
Also, a second object of the present invention is to carry out sufficient desulfurization even when the content of mercaptans in an impurity gas (acid gas) is high, and to improve the recovery rate of sulfur in the overall process.